In a conventional optical information recording and reproducing system, an optical disk provided with a spiral track composed of a series of pits is used, and a reproduced light beam obtained by projecting of a laser beam onto the optical disk is directed to a photodetector, thereby resulting in that a reproduced signal is detected by the photodetector. So, the conventional optical information recording and reproducing system has limits in its information recording density due to (1) a pit interval on each recording track and (2) a pitch of the recording tracks.
There is known a method of optically recording and reproducing of information which considers the abovementioned deficiency, the method being disclosed for example in the Japanese examined patent publication No. 4-37486/1992 (Tokukohei 4-37486). According to the method, as shown in FIG. 11, diffused light beams from a semiconductor laser 31 are directed to an objective lens 35 through a collimator lens 32, a polarizing beam splitter 33, and a quarter wave plate 34 so as to project converged light beams to an optical disk 30. Reflected light from the optical disk 30 is directed to the polarizing beam splitter 33 through again the objective lens 35 and quarter wave plate 34, and the reflected light is deflected by 90.degree. so as to direct to a multiple hologram 36 (described later). The light beam incident on the multiple hologram 36 is a reproduced object wave beam in accordance with recorded pit patterns which are preliminarily formed on the optical disk 30. So, a reproduced reference wave beam is projected from the multiple hologram 36 to respective photodetectors 37a to 37d which correspond to the respective pit patterns, the reproduced reference wave beam having a projecting angle which varies depending on the kind of pit pattern. When it is assumed that there are, for example, four kinds of pit patterns as shown in FIGS. 12(a) to 12(d), total four photodetectors 37a to 37d are required accordingly. The beam incident on the respective photodetectors 37a to 37d is strongly directed to one of the photodetectors corresponding to the pit pattern. According to the detection that which detected output level is greatest among those of the photodetectors 37a to 37d, it can be judged which pit pattern on the optical disk 30 is reproduced. The transmitted light from the multiple hologram 36 is arranged by a convex lens 38 so as to form a converged light beam. The converged light beam is directed to a photodetector 39 for tracking and forcussing-uses. Servo signals are generated by the photodetector 39.
The following description deals with a manufacturing method of the multiple hologram 36 corresponding to the optical disk 30 with reference to FIG. 13.
First, a laser beam is directed to a beam splitter 40 from a light generating source (not shown). One of the split light beams by the beam splitter 40 is projected, through a polarizing beam splitter 41, a quarter wave plate 42, and an objective lens 43, onto the pit patterns preliminarily formed on the optical disk 30. Thereafter, a reproduced light beam, which is reflected from the pit pattern, is directed to the polarizing beam splitter 41 through the objective lens 43 and the quarter wave plate 42. The polarizing beam splitter 41 deflects the reproduced light beam by 90.degree. so as to direct to the multiple hologram 36 as the object wave beam.
The other one of the light beams split by the beam splitter 40 is reflected by a mirror 46 through a half wave plate 44 and a convex lens 45 so as to direct to the multiple hologram 36 as the reference wave beam. In such case, the position and inclined angle of the mirror 46 changes in accordance with each pit pattern on the optical disk 30 such that the point, on which the image is formed when the reflected light beam is assumed to transmit the multiple hologram 36, coincides with the position where the detector for detecting the pit pattern is deposited. The interference fringes between the object wave beams in accordance with the respective pit patterns and the respective corresponding reference wave beams are in multiple recorded on the multiple hologram 36.
The conventional optical information recording and reproducing system can reproduce the four kinds of information from one recorded area. However, the pit paterns can be adapted only to a so-called CD-type apparatus for playback-use only which carries out reproducing in accordance with the preliminarily recorded shapes such as convex and concave. So, the problem arises that the abovementioned conventional system can not be directly adapted to a rewritable recording and reproducing apparatus such as a magneto-optical disk apparatus wherein the recording is carried out based on the m gneto-optical effects.